The present invention relates to a chip-type common mode filter. More specifically, the present invention relates to miniaturized chip-type common mode EMI filters with a novel design so that they can be manufactured very economically with substantially reduced physical dimension, while maintaining high common mode impedance, low normal mode impedance, so as to maintain a high fidelity of normal mode waveforms and minimize common mode noises for devices that utilize differential transmission technology, The common mode filter developed in the present invention is most advantageous for the manufacturing of the so-called xe2x80x9clow profilexe2x80x9d electronic devices such as notebook and handheld computers or other portable consumer electronic goods. The present invention also relates to the method of making the novel chip-type common mode filter.
It is well known that electronic instruments produce electromagnetic interference (EMI), which can interfere with the operation of other electronic instruments. Conventionally, EMI is modeled as two superposed current components: common mode current and differential mode current. By definition, differential mode currents travel in opposite directions on a pair (or multiplicity) of conductors. And common mode currents are defined as traveling in the same direction on a pair (or multiplicity) of conductors. Differential-mode noise travels in the same direction as the desired signal. In contrast, common mode noise is expressed as a displacement with respect to a reference potential on both (or a multiplicity of) conductors at the instrument location. If the instrument in question measures its signals with respect to a ground potential, common mode noise may corrupt its measurement of the desired signal. Rejection of all common-mode currents, i.e., noise, is desired.
The need to remove common mode EMI becomes more imperative in light of today""s rapid development in internet communication and digital data transmission devices which involve constantly increasing data transfer rates. Another problem facing the industry today is the wide use of the differential transmission technique in various devices, such the universal serial bus (USB), IEEE 1394 interface standard, low-voltage differential signaling (LVDS), etc. Because differential transmission involves pairs of transmission lines, so as to provide good a return route for each signal, the EMI noises are primarily common mode noises. The frequency interval of the EMI noises often overlaps with the frequency interval of the signals themselves. This makes it difficult for the conventional EMI filters to effectively reduce the common mode EMI noise without adversely affecting the quality, and/or fidelity, of the normal mode portion of the differential transmission signals.
To satisfy this need, as well as to accommodate the decreased size of today""s electronic circuits, a number of common mode choke coils have been developed which are formed by winding two conductors on a ring-shaped magnetic core or bar-shaped magnetic core the same number of turns in the same direction. U.S. Pat. No. 5,552,756, the content thereof is incorporated herein by reference, discloses a chip type common mode choke coil which is, compared to previous filters, relatively simple in structure, suitable for miniaturization, high in mass productivity, and high in economization. The common mode choke coil contains a plurality of non-magnetic sheets on which a plurality of double-loop conductor lines (or coils which bend at approximately a right angle) are formed, respectively. A pair of magnetic layers are formed above and below the upper and lower surfaces, respectively, of the plurality of the non-magnetic sheets. Finally, a pair of magnetic cores are arranged substantially at the centers of the loops of the conductor coils so as to form a closed magnetic path.
The common mode choke coil disclosed in the ""756 patent made the first step towards the miniaturization of common mode filters. However, because it requires the placement of the multiple sheets in order to provide the alternately connected double-loop conductor coils and the pair of magnetic cores, the manufacturing process is still not simple enough, and the structure not compact enough. As a result, its manufacturing cost may not satisfy the expectation of today""s consumers. Furthermore, it may not be compact enough.
The primary object of the present invention is to develop an improved common mode filter with substantially reduced size, simplified construction, low normal mode impedance (Zn) and high common mode impedance (Zc). More specifically, the primary object of the present invention is to develop an improved and miniaturized common mode EMI filter with a greatly simplified design so that it can be manufactured very economically, while retaining low normal mode impedance and high common mode impedance, and with a substantially reduced physical size, so that it can cost-effectively maintain a high fidelity of the normal mode waveform of signals for electronic devices that utilize differential transmission technology and keep the common mode EMI noise to a minimum. The common mode filter of the present invention, which contains a pair of planar coils embedded in a magnetic substrate with excellent coupling and well balanced electric inductance, is most advantageous for use in making notebook and handheld computers, as well as other portable electronic devices.
The chip-type common mode filter disclosed in the present invention contains a magnetic main body and a pair of substantially identical electrically conducting planar coils embedded in the magnetic main body. The pair of electrically conducting planar coils are intimately (i.e., touching each other) separated by a substantially identical insulative planar coil, which is also embedded in the magnetic main body. More specifically, the chip-type common mode filter disclosed in the present invention contains the following elements:
(a) a magnetic main body;
(b) a pair of substantially identical electrically conductive planar coils embedded in the magnetic main body; and
(c) an insulative planar coil sandwiched between the pair of electrically conductive planar coils, wherein the insulative planar coil has a pattern that is substantially identical to and inclusive of the pattern of the electrically conductive planar coils so as to insulate the electrically conductive planar coil from each other.
In order to effectively insulate the electrically conductive planar coil from each other, the insulative planar coil should preferably have a width that is wider than the width of the electrically conductive planar coil so as to provide effective insulation. Preferably, the electrically conductive planar coils are multi-loop (at least two loops) planar coils having a spiral, serpentine, or meandering pattern. During the process of fabricating the common mode filter of the present invention, the magnetic substrate grows through the void spaces of these loops as well as completely around them. The embedding of the electrically conductive planar coils, which are electrically insulated from each other by the insulative planar coil, allows a close-looped magnetic field to be generated which encompasses the cross-sections of the stacked planar coils and is perpendicular to the direction of the running direction of the stacked (or laminated) planar coils.
The chip-type common mode filter of the present invention also contains four electrical terminals, first input and first output, and second input and second output, all of them are partially embedded in and partially exposed from the magnetic main body for providing electrical connections to the pair of electrically conductive planar coils, respectively. The electrical terminals can be part of the electrically conductive planar coil (i.e., on the same plan), or they can be provided as separate relatively short planar coil segments which are disposed at different plans but are connected to the electrically conductive planar coils, respectively at the inner ends thereof. A similarly shaped insulative planar coil segment is utilized to separate the conductive planar coil from the conductive planar coil segment, except their inner end portions. In this embodiment, the end portions of both the conductive planar coil and the conductive planar coil segment will be pressed against each other by pressure exerted from the magnetic main body during the lamination process, to provide electrical connection.
In another embodiment, two stacks of the coils (i.e., two stacks of conductive-insulative-conductive planar coils) are further stacked on top of each other, but are separated by a partitioning magnetic layer to form serially connected filters. The conductive planar coils in one stack are connected to corresponding conductive planar coils in the second coil stack by conductive vias (i.e., filled conductive holes), respectively, that are formed through the partitioning magnetic layer. These cause two individual common mode filter circuits to be connected in series. Additional filter circuit or circuits can be firther connected to form further multiple common mode circuits.
Preferably, the common mode filter circuit of the present invention is made using a thick film (such as printing) or thin film (such as sputtering) technology. For the thick film process, it can comprise the following steps:
(a) forming a first electrically conductive planar coil on a bottom of a magnetic green sheet, followed by formation of an insulative planar coil and a second substantially identical electrically conductive planar coil sequentially, on top of the first electrically conductive planar coil to form a common mode circuit;
(b) placing a top magnetic green sheet on the second electrically conductive planar coil; and
(c) sintering the top and bottom green sheets with the coil stack embedded therein to form a chip-type common mode filter.
If a thin film process is used, the magnetic green sheet is replaced with a magnetic substrate, and the steps of placing the magnetic green sheets is replaced with the step of sputtering top and bottom magnetic films on the substrate. No sintering step is necessary for the thin film process.
The magnetic main body mainly comprises ferrite, which contains Fe2O3, NiO, CuO, ZnO, MnO2, CoO, etc. The electrically conductive planar coils can be made of a conductive paste or sputtering metal, containing mainly silver, copper, or alloys of silver and palladium. The insulative planar coil can be magnetic or non-magnetic. It can be made of ferrite (i.e., the same material as the material making up the magnetic main body), or a mixture of ferrite and non-magnetically permeating material conducting such as MgO, SiO2, B2O3, PbO, ZnO, Al2O3, etc. If the insulative planar coil is made of a mixture of the magnetically permeating material and the non-magnetically permeating material, its magnetic conductivity can be adjusted to vary between 1 and that of the magnetic main body.